Stimulated Raman scattering processes have been applied recently in the development of tunable spin-flip Raman lasers such as disclosed by A. C. K. N. Patel and E. D. Shaw, Physical Review B3, 1279 (1971). Typically, previous devices consisted of focusing the optical pumping radiation from either a carbon monoxide laser at 5.mu.m wavelengths or a carbon dioxide laser at 10.mu.m wavelengths into a crystal of indium antimonide (InSb) which is held in a cryostat at a temperature of about T= 1.degree. to 30.degree. K; a variable magnetic field is applied to the indium antimonide crystal to produce and change the frequency of the spin-flip Raman laser output. The Raman scattering material which may be indium antimonide crystals shaped as a parallelepiped with dimensions of a few millimeters along each edge. At least two opposite sides are polished planes and parallel to form a Fabry-Perot type optical resonator. Generally, the polished sides are chosen to be those through which the pumping radiation enters and leaves the crystal.
Devices constructed in this manner suffer from the disadvantages that the pumping source, such as the beam from a CO or CO2 laser, must be focused to a small spot to achieve sufficiently high irradiance in the scattering material to exceed the threshold for stimulated Raman scattering. The irradiance is limited by the focal length of the lens or mirror which is determined by the minimum distance that it can be positioned from the scattering material. Generally the focusing optic must be several centimeters. Also the resonators are constructed with uncoated external surfaces to provide a reflectance of 35%. The incident pumping radiation is thereby reduced 35% by this reflectance at the entrance surface after traversing the length of the scattering material, the internal irradiance is further reduced 65% by transmission through the exit surface. The internal irradiance is distributed throughout the volume of the plane-parallel resonator because of multiple reflections of the boundary surfaces. Each of these conditions effects a reduction in the internal irradiance which requires higher incident power in the pumping beam to exceed the threshold for stimulated Raman scattering. The plane-parallel type resonator also is extremely susceptible to high radiation losses because of misalignment of the end surfaces and defraction of the resonant wave front.